(1) Field of the Invention
The present invention relates to a fixing device and an image forming apparatus, and in particular to a fixing device and an image forming apparatus provided with the fixing device that have a structure allowing for switching of a pressing member between a pressing state and a separated state with respect to a heating rotating body of a fixing roller or the like.
(2) Description of the Related Art
In image forming apparatuses such as printers, a fixing device that uses a heat fixing method fixes toner to a recording sheet in the following way. For example, a pressing roller, which acts as a pressing member, is pressed against the circumferential surface of a rotating fixing roller by the restorative force of an elastic member such as a compression spring, thereby forming a fixing nip. A recording sheet carrying a toner image is passed through this fixing nip in order to fix the toner image to the recording sheet.
The outermost surface of the pressing roller is typically an elastic layer formed from silicone rubber or fluorinated resin. A portion of the elastic layer elastically deforms in order to form the fixing nip. As a result, if the pressing roller is continually pressed against the fixing roller, the portion of the elastic layer that elastically deforms may not fully return to its original shape if a long time passes without any images being formed. Such a change in shape would prevent smooth transport of recording sheets.
One way of addressing this problem is to provide a switching mechanism (a switching unit) that switches the pressing roller and the fixing roller between a separation state, in which the pressing roller and the fixing roller are not in contact, and a pressing state in which the pressing roller presses against the fixing roller. The switching mechanism places the pressing roller and the fixing roller in the separation state by resisting the restorative force of the elastic member during any time other than image formation (fixing) and places the pressing roller and the fixing roller in the pressing state by allowing the restorative force to act during image formation.
The switching mechanism may, for example, include a plate cam and operate by receiving the rotational power of a motor transmitted over a power transmission mechanism that includes a gear train or the like. The circumferential surface of the plate cam abuts a frame or the like that supports the pressing roller. As the plate cam rotates, the pressing roller is separated from the fixing roller, against the restorative force of the elastic member, or is returned to a position so as to press against the fixing roller.
There is a desire to reduce the cost of all manufactured goods, and image forming apparatuses are no exception. In particular, there is a desire to reduce the cost of the fixing device, which is relatively high as compared to other components of an image forming apparatus.
One approach that has been examined to reduce costs is to reduce the number of motors by one by using the same motor as both the motor for the switching mechanism and the rotary drive motor for the fixing roller. Hereinafter, the motor shared by the fixing roller and the switching mechanism is referred to as a shared motor.
For example, a clutch may be incorporated in the power transmission mechanism between the shared motor and the cam (switching mechanism), and rotation of the cam can be controlled by engaging and disengaging the clutch before and after a series of image forming operations.
Such a structure, however, causes the following problems to arise.
When separating the pressing roller, the shared motor must bear not only the load for compressing the compression spring, but also the load for rotary driving of the fixing roller. As a result, a shared motor with a high torque must be used, which increases the size of the motor, thus increasing the size of the fixing device.
In order to address this problem, the load (torque) on the shared motor can be reduced when the shared motor separates the pressing roller (i.e. compresses the compression spring) by setting a high reduction ratio for the power transmission mechanism. Setting the reduction ratio to be high, however, slows down rotation of the cam, causing an increase in the time necessary to return the pressing roller to the pressing state (in which the pressing roller presses against the fixing roller). The result is a longer time before image forming (fixing) begins for the first sheet being printed.